Sunday, 9 October 2011

Bats Can Be Colourful, Too

Spotted bat, Euderma maculata

It's stating the obvious to say that mammals have a range of different colours and coat patterns. The purpose of all these different markings can be varied: they may help to identify members of particular species to their kin, they may be used for sexual attraction, they can act as camouflage, and so on. Nor is there any particular reason to suppose that a given pattern has to serve only a single function. There are a great many colourful mammals, but bats are generally not among them.

The bats are the second largest order of mammals, after the rodents, including a total of nineteen different families - most of them with really obscure names - and well over a thousand different species. Yet, despite this great diversity, most bats are pretty much the same colour all over - usually a variation on the theme of "it's brown". But not all of them; many bats are surprisingly colourful, and you might wonder what the point of that is if they only come out at night, and spend the rest of the day sleeping in pitch black caves. The question really is not so much why are most bats so bland, but why aren't they all that way?

Except, of course, that not all bats do live in caves, and that may well have something to do with it. For the second largest order of mammals, bats have not been as well studied as most other groups. Most of the studies that have been conducted have tended to focus on the undeniably cool fact that bats navigate using sonar. Yet they are a very interesting, and one might even say peculiar, group of mammals. It may be, for example, that just looking at the colours of bats can tell us something about the reasons for coat patterns in mammals in general. A recently published survey by Sharlene Santana of UCLA, and colleagues, examined published descriptions of over nine hundred species of bat, cross-checking the patterns of their fur with their lifestyle. Was there... well, a pattern to the patterns?
The first thing to note is that, yes, most bats are quite bland in terms of their fur colour. Over 80% of the species surveyed are essentially the same colour all over, with no contrasting patterns or shading. Indeed, some families of bats include no species with markings at all. On the other hand, those families are generally very small, with only a handful of species, all of which are, even by the standards of bats, living in much the same way as each other. When it comes to the really large and diverse families of bats - especially the big three of vesper bats, leaf-nosed bats, and flying foxes - while most of them are still bland, we do see a greater variety of coat pattern among those that aren't.

Perhaps the most common colour pattern among mammals in general is a sort of basic counter-shading. The animal is more or less the same colour over most of its body, but it has paler underparts, sometimes quite clearly separated from the darker fur above, but often less so. We see this pattern, for example, in many rodents, but also in a range of species across the mammals. The usual explanation for this is that mammals, being (usually) four-legged, tend to have a horizontal posture. When you see the animal from the side, it's body casts a shadow, so the paler underparts appear visually to be the same sort of shade as the more illuminated upper parts. Which is handy if you're trying to camouflage yourself from predators, for example. Or, if you are a predator, if you don't want your dinner to see you coming.

It turns out that, in bats, this normally common colour pattern is actually quite rare. Less than 5% of the species surveyed had this pattern, at least according to published descriptions and any available photographs. But then, bats don't walk along the ground on four legs. For those that roost in caves, shadows really aren't an issue, and those that roost outdoors aren't doing so horizontally, but are instead hanging head down. In that context, its interesting to note that, while it's still a rare pattern, some tree-roosting bats, especially among the flying foxes, have pale markings on their shoulders and around their neck. If you're hanging upside down from a branch, and light is filtering through the leaves up above, that is exactly where the shadows are going to be, so this may serve the same purpose as more typical counter-shading in other species.

Far more common, however, are those bats with stripes or spots on their bodies, faces, or wings. Sometimes these can be quite colourful and distinctive, as much as those on many more familiar animals. The usual explanation for such bold patterns on animals is that it helps to break up the animal's outline, acting as a kind of camouflage that makes it difficult for predators or prey to easily follow, especially when it's moving rapidly. Zebras and tigers are obvious examples, here. Of course, as I mentioned above, such colours may also be used to signal to other members of their own species, for example, to indicate reproductive fitness. Or, in some cases, even signals to other species; in the case of skunks, the highly contrasting colours seem to be a warning to would-be predators to stay the heck away if they don't want a face full of stink.

Bats have pretty good eyesight, but that's of no use in the pitch blackness of caves. Equally, anything that's going to eat you in a cave won't be confused by contrasting stripes, as it has to be hunting by sound or scent anyway. So, regardless of whether they're using the patterns as camouflage, or as means of identifying each other, we might expect that striped and spotted bats are not the sort to live in caves.

And this is what the survey revealed: cave-dwelling bats tend to be the blandest in appearance, while the most colourful bats tend to roost in vegetation, and not underground. It's not a hard and fast rule, so there may be other factors at play, but there is a definite tendency. Indeed, the bats that just roost in trees by hanging from branches, where they will be hidden by a few conveniently placed leaves at best, are more likely to be striped or spotted than those that go to a bit more effort to hide themselves. For example, some bats actually construct tents out of leaves, so that they are only visible from below. There is some evidence that these bats tend to have stripes on their faces, the only readily exposed parts of their bodies when they're sleeping, although, since we're not talking about a lot of species here, it's hard to prove that that means anything.

So, are these patterns mainly for camouflage? Given the number of species we're talking about, it's unlikely that the answer is always so simple, but there is another indication that it's a pretty major factor. That's because it's not just outdoor roosting bats that are more likely to be coloured, its also those that live in smaller colonies. Bats that live in exceptionally large colonies are at less risk, individually, of being eaten by predators, and they also tend to be larger animals, which means that there are fewer predators able and wanting to eat them anyway. Of course, many of the really big colonies of bats are found in cave-dwelling species, where the colonies may be more populous than most human cities, but the pattern still seems to hold among the more "outdoor" species.

That's not to say that bats can't use coat colours to signal to each other, and at least some of them certainly do. But there's no reason that they can't do both. For example, a solitary, tree-dwelling bat might evolve stripes to hide itself from predators when pressed against the bark, and then also use the same pattern to identify sexual partners when it has to go out and look for some. But, nonetheless, it seems that, if you're a bat, the best way to hide is not to be a bland, boring colour.